Apple Watch Series 2 not saving swim data

I have recently started using my Apple Watch to record open water swimming. It worked fine the first time and saved all the data, but on the last 3 occasions it is not taking me to the data page in order to save the activity. The clock is always still running, but when I press end, It just returns to the main page to select a workout and subsequently loses all my workout data. It works fine for all other activities - running, cycling, walking, HIIT, yoga. Can you help?